This site list magnum girth as 5.5" circumference and 1.75" diameter. This is not true - I just measured mine at 4.75" circumference at the base of the shaft and at the head, and even thinner around the middle. So unless you're attempting to insert the flared base, this is not a particularly girthy toy.
I got confused about this when I started looking for something a bit thicker than my magnum, looked at the Mikey O2, and it's listed as having the same girth! Not so!